The present invention relates generally to metal containers and more specifically to the manufacture of drawn, extruded and/or ironed containers. In the formation of these containers, a flat blank is severed from a continuous sheet of stock material and is formed in several steps to produce a container that has a cylindrical sidewall and a bottom or end wall integral with the sidewall and formed to a particular configuration to increase resistance to pressure interally of the container.
One container that has received a remarkable degree of attention is the type that is disclosed in U.S. Pat. No. 3,942,673, which is assigned to the assignee of the present invention. This patent discloses a drawn and ironed container that has a particular configuration between the sidewall of the container and an ellipsoidal dome which forms the major part of the bottom wall. In the formation of a container of this type, a flat blank is normally drawn to an intermediate configuration which includes a cylindrical sidewall and a flat bottom wall which is integral with the sidewall through an inclined peripheral flat wall. The container is then formed to its final configuration through a pair of dies that respectively cooperate with inner and outer surfaces of the container.
In the formation of drawn and ironed containers of the type under consideration, it has been highly desirable to form the die member that is received inside of the container as a "split punch design." In the formation of the split punch design, a female portion formed of one material cooperates with a generally cylindrical member to form the cooperating die that is received inside the container during the final formation of the bottom wall.
With this arrangement the ironed and domed container can be positively removed from the punch assembly by separation of the cylindrical member or punch sleeve from the female portion or nose piece at the end of the punch stroke.
This means that the female portion or nose portion of the die can be formed from a metal, such as steel, while the cylindrical portion of the die, which engages the sidewall of the container can be formed from a material such as carbide. It has been found that the use of carbide for the major portion of the die results in better stripping of the die from the container after it has been ironed and longer life for the dies as well as a better surface finish on the inner surface of the container which, therefore, requires less subsequent coating before the container is filled with product.
In the formation of the split punch design, it has been customary in the past to have the adjacent surfaces of the nose portion and the sleeve located at an angle with respect to the axis of the container so that the nose portion and sleeve can easily be split. It has been determined that utilizing a surface that is inclined with respect to the axis of the container as the mating surface between the two members on the periphery thereof simplifies the matching of the two surfaces.
However, it has also been determined that such a mating arrangement creates other problems which reduces the service life of the unit. For example, it has been found that the two inclined surfaces that define the mating surface between the punch and the nose wear rapidly, which will result in having the periphery of the two members at the joint being offset from each other. This arrangement results in producing an undesired deformation or sharp line in the area between the sidewall and the bottom wall. This wearing of the two surfaces is further aggravated by the fact that there is relative movement between the two surfaces each time the nose portion and the punch portion are engaged with each other.
Another problem encountered with the split punch design as has previously been utilized is that, when a jam or other malfunctioning of the press occurs, it results in misalignment of the peripheral edges of the two members and can cause chipping of the sharp point on the lower end of the carbide member. Thus, manufacturers must constantly replace the entire unit which is fairly expensive procedure. It will be appreciated that the cost of a cylindrical member formed of carbide is extremely high and any changes that can be made to increase the service life thereof results in substantial savings in manufacturing costs.